During epithelial-mesenchymal interactions in numerous embryonic epidermal organ systems there is an assumed transfer of developmental information between tissues. This has been indirectly confirmed by heterotypic cell recombinant experiments, transfilter induction experiments and the diffusion of various isotopic precursors from one tissue to the other in vivo and in vitro. We described the fine structure of matrix vesicles found in the extracellular matrix interposed between tooth forming epithelia and mesenchyme. Some of these matrix vesicles contain RNAs which enhance cytodifferentiation in vitro. We isolated matrix vesicles during embryonic and early postnatal incisor tooth development. Are vesicles a morphological basis for transfer of developmental information? What are the biological functions for intercellular vesicles? In which direction do they migrate? The employment of two congenic strains of genetically defined mice (B10.D2(H-2d) and C57BL/10Sn(H-2b), differing only in the gene locus, provides a means to selectively make alloantibody against histocompatibility alloantigens located on outer cell surfaces of embryonic cells. Antibodies labeled with fluorescein or ferritin can be used to selectively label either epithelial or mesenchymal membranes during experiments designed to mix the two tissues, each derived from a different strain of mice. Assuming that matrix vesicles synthesized within either tissue type acquire a membrane "marker" specific to the strain of mice (haplotype) or if embryonic cells and matrix vesicles acquire "differentiaion alloantigens," one can observe the actual transfer of material with immunogenetic techniques. Subsequently, material actually transferred from tissue to tissue can be recovered, analyzed and studied as to mechanisms of action. Understanding developmental instructive events is imperative during craniofacial growth and development. BIBLIOGRAPHIC REFERENCES: Lehmann, R. and H.C. Slavkin (1976) Localization of "Transcriptively active" cells during ondontogenesis using acridine orange ultrastructural cytochemistry. Dev. Biol. 49:438-456 (5 reprints enclosed). Slavkin, H.C., and P. Bringas (1976) Epithelial-Mesenchymal interactions during odontogenesis: Direct heterotypic cell-cell contacts. J. Dent. Res. 55: Spec. Issue B: 626 abstract.